Unburned zone



March 31, 1964 F. E. CAMPION 3,126,954

IN SITU COMBUSTION PROCESS Filed Dec. 30, 1959 INJECTION EL GASES FLUIDSBURNED ZONE UNBURNED ZONE FIG. I

INJECTION PRODUCED GASES FLUIDS UNBURNED ZONE BURNED ZONE UNBURNED ZONEFIG. 2

Francis E. Compioninventor B t. Attorney United States Patent Office3,126,954 Patented Mar. 31, 1964 3,126,954 IN SITU COMBUTHQN PRUQESSFrancis E. Campion, Tuisa, @ida, assignor to Jersey Production ResearchCompany, a corporation of Delaware Filed Dec. 36, 1959, Ser. No. $62,9438 Claims. (Cl. 166ll) The present invention relates to thermal methodsfor the recovery of petroleum from underground formations and moreparticularly relates to an improved process for recovering oil frompartially depleted reservoirs by the in situ combustion of carbonaceousmaterials present in such reservoirs. In still greater particularity,the invention relates to an improved in situ combustion process whereinmaximum utilization of oxygen in the input gas stream followingbreakthrough of the initial combustion front at the production Well isachieved by establishing and maintaining a secondary combustion front inthe reservoir between the injection and production Wells.

In situ combustion is an attractive method for recovering oil frompartially depleted underground reservoirs. This method in essenceinvolves the establishment of a combustion front within the reservoir inthe vicinity of one or more injection wells and the subsequentintroduction of a combustion-supporting gas behind the combustion frontin order to move it through the reservoir toward one or more productionwells. As the combustion front advances, the heat liberated results inthe vaporization of oil from a high temperature zone preceding thefront. Cracking and the formation of coke which serves as fuel for theprocess also occur. The resulting oil vapors are carried forward withthe combustion products and condensed in cooler portions of thereservoir. Heat transfer to cold oil in sections of the reservoir infront of the advancing high temperature zone leads to a reduction inviscosity of the oil and facilitates its displacement from thereservoir. A mixture of oil and gases is withdrawn from the reservoir atthe production Will and the oil contained therein is subsequentlyrecovered.

Although this process is a promising one, there are several difficultiesassociated with it. Not the least of these is the fact that thecombustion front and associated high temperature zone often tend tofinger through high permeability sections of the reservoir instead ofadvancing uniformly. This frequently leads to breakthrough of thecombustion front into the production well at an early stage in theprocess. After such a breakthrough occurs, high temperature combustionproducts, oxygen and oil flow into the combustion well simultaneously,creating a danger of fire in the wellbore and in associated equipment.Process efficiency in terms of the utilization of injected oxygen fallsoff and the operation becomes less attractive from an economicsstandpoint. Despite this, the introduction of oxygen at the injectionwell and the withdrawal of fluids from the production well must becontinued if additional oil is to be recovered from the reservoir.

Difficulties of the type described above are encountered in bothStratified and unstratified reservoirs. In Stratified reservoirs, someof the strata frequently have much greater permeability than others.Breakthrough of the combustion front at the producing well through ahigh permeability stratum results in a direct channel through which theinjected oxygen may pass from the injection well to the production wellwithout encountering significant quantities of unburned hydrocarbons.Recovery from the adjacent strata of relatively low permeability is slowand must be carried out in the face of undesirably high temperatures andfree oxygen in the production Well.

In unstratified reservoirs, breakthrough generally occurs at theproduction well near the top of the producing formation due tooverburning. Utilization of the injected oxygen thereafter dependsprimarily upon the diffusion of gaseous oxygen from the burned out areaat the top of the reservoir down into the unburned zone. The distancethrough which the oxygen must diffuse increases as the thickness of theburned-out zone increases and hence the portion of the injected oxygenutilized to support combustion is apt to decline as the operationprogresses. The result is a continual increase in the difiicultiesoccasioned by the presence of free oxygen in the production well.

The present invention provides a new and improved in situ combustionprocess which largely obviates the difficulties outlined above. Inaccordance with the invention, it has now been found that the problemsencountered following breakthrough of the combustion front at theproduction well during in situ combustion can largely be avoided byestablishing and maintaining a secondary combustion front in thereservoir at an intermediate point between the injection and productionwells after breakthrough occurs. Such a secondary front results insubstantially complete utilization of the injected oxygen within thereservoir. The heat liberated due to combustion at the secondary frontis transferred to relatively impermeable sections of the reservoir moreeffectively than is otherwise possible. Total oil recovery is increased.The invention thus provides an improved in situ combustion process whichis safer, more effective and more economical than in situ combustionprocesses employed in the past.

The secondary combustion front utilized in practicing the invention isestablished at an intermediate point in the reservoir between theinjection and production wells after breakthrough of the initialcombustion front by introducing a preselected mixture of fuel gas,oxygen and inert gas at the injection well. The mixture thus introducedflows through the high permeability, burnedout section of the reservoirtraversed by the initial combustion front, picking up heat as itprogresses. When the ignition temperature of the fuel gas in the mixtureis reached, generally after the mixture has advanced a relatively shortdistance into the reservoir, combustion occurs. The oxygen and fuel gasinjected thereafter are consumed at the secondary combustion front thuses tablished and hence never reach the initial combustion front.Combustion of coke at the initial front and the how of free oxygen intothe production well cease. Heat liberated at the secondary front is inpart transferred to the surrounding rock and in part carried todownstream sections of the reservoir by the combustion products andinert gases. This heat stimulates the production of oil from relativelyimpermeable zones by-passed by the initial front. Temperatures at theproduction Well are considerably lower than in operations not utilizinga sec ondary front.

The gaseous mixture introduced at the injection well in order toestablish and maintain the secondary combustion front utilized in thepractice of the invention will normally comprise a fuel gas, oxygen andone or more inert gases. Suitable fuel gases include methane, ethane,propane, butane, natural gas, mixed refinery gases and the like. Inertgases which may be employed as diluents include carbon dioxide, nitrogenand flue gases. A typical gaseous mixture may contain from about 1 toabout 10 volume perment fuel gas, from about 3 to about 25 volumepercent oxygen, and from about 65 to about 96 volume percent inert gas.It is generally preferred to mix the fuel gas with air to provide thenecessary oxygen and to add sufiicient inert gas to give the desiredmixture. The relative amount of each constituent utilized, of course,depend, to some extent upon the particular fuel 3 gas used and, aspointed out below, will also depend in part upon the point at which" thesecondary front is to be maintained.

After the secondary combustion front has been esta lished, thecomposition of the gas stream introduced into the reservoir through theproduction well is controlled to secure high oil recovery rates. Sincethe heat transmitted to oil bypassed by the initial combustion frontdepends in part upon the point at which it is liberated within thereservoir, the recovery rate can be regulated by controlling theposition of the secondary front. Studies and experimental work haveshown that the position depends largely upont he oxygen-to-fuel ratio inthe gaseous mixture introduced at the injection well. An increase in theratio generally results in movement of the secondary front back towardthe injection well; while a reduction normally causes it to move forwardtoward the production well. Proper adjustment of the ratio at intervalspermits maintenance of a substantially stationary front. By thus varyingthe relative amounts of oxygen and fuel in the gas stream injected, theposition of the secondary front can be controlled to obtain highrecovery rates without encountering unduly high temperatures and freeoxygen at the production well.

The oxygen-to-fuel ratio required to establish and maintain thesecondary combustion front at an intermediate position between theinjection and production wells will, as pointed out above, dependsomewhat upon the particular fuel gas utilized. It will also depend uponthe physical properties of the reservoir, the fluids present therein,and the course taken by the initial combustion front prior tobreakthrough at the production well. These latter factors are extremelydifficult to evaluate during an in situ combustion operation and henceit is impractical to attempt to prescribe the precise oxygen-to-fuelratio which will be required in a particular situation. A moresatisfactory method for controlling the operation is to initiallyestablish the secondary front by injecting a combustible gaseousmixture, one containing theoretically equivalent amounts of fuel gas andair under atmospheric conditions for example, and thereafter to adjustthe composition until a satisfactory recovery rate is attained. Any ofthe conventional methods of measuring the recovery rate may be utilized.There is generally an appreciable time.

lag between changes in the input gas composition and changes in therecovery rate and hence it is normally preferred that a day or moreelapse between changes in composition so that the effect of each changecan be fully assessed. Measurement of the temperature and composition ofthe gases produced at the production well serves to indicate that thesecondary front has been established and that combustion at the initialfront has ceased.

fter the secondary front has been established and a satisfactory oilrecovery rate has been attained, it is preferred that the oxygen-to-fuelratio be held constant until a decline in the recovery rate indicatesthe heat transferred to the oil-bearing section of the reservoir is toolow for most effective recovery. At that point, the secondary front ismoved forward in the reservoir by reducing the oxygen content of theinput gas stream slightly. Such a change in the position of the frontresults in the liberation of heat nearer the oil-bearing region in thereservoir and, since more heat is transferred to the oil present there,produces an increase in the recovery rate. The position of the secondaryfront may be continuously controlled in this manner to obtain maximumoil recovery.

In addition to controlling the position of the secondary combustionfront, it is generally desirable to regulate the mass rate at which thegases are injected into the reservoir. This provides a further controlon the temperature of the fluids flowing into the production well. Ifthe production well temperature is too high, the mass rate can bereduced to reduce the total heat input per unit of time. The energycarried to the production well as sensible heat in the product gaseswill therefore decline. if, on the other hand, an increase in thetemperature of the product flowing into the production well is desired,the mass rate may be increased. The actual quantity of gas injectedwill, of course, partially depend upon the porosity and permeability ofthe reservoir and upon the reservoir pressure. In general, pressuresbetween about pounds per square inch and pressures approaching theoverburden pressure may be used.

The method of the invention is applicable to a variety of differentoil-bearing reservoirs wherein breakthrough of the initial combustionfront occurs at the production well before recovery of the oil containedin the reservoir is completed. It may be applied to processes whereinreverse burning is utilized as well as to conventional in situcombustion operations. The method is not limited to the use of a singleinjection well and a single production well and instead may be carriedout using both multiple injection wells and multiple production wells.Any of the various systems conventionally utilized for spacing theinjection and production wells may be utilized.

The nature and objects of the invention can be more"- fully understoodby referring to the following detailed description of in situ combustionprocesses carried out in accordance therewith and to the accompanyingdrawings in which:

HS. 1 depicts an in situ combustion process utilizing a secondarycombustion front wherein overburning occurred prior to breakthrough ofthe initial combustion front at the production well; and,

FIG. 2 represents a process wherein a secondary combustion front is usedin a stratified reservoir after breakthrough of the initial frontthrough a high permeability zone.

Referring now to FIG. 1, reference numeral 11 desig nates an injectionwell drilled through overburden 12: into an oil-bearing reservoir 13.Production well 14- has been drilled into the reservoir at a pointremovedfrom the injection well. The distance separating the injectionand production wells will depend upon a number of factors, including theextent of the reservoir, the permeability andporosity of the subsurfacestrata, the reservoir pressure, and the recovery pattern utilized. Thisdistance may vary widely but will generally range between about 300 feetand about 3,009 feet. The injection and production wells will normallybe cased and perforated opposite the producing strata in theconventional manner but in some instances uncased wells may be used.Conventional f cilities for introducing air and other gases at theinjection well and for recovering and separating oil and gaseousproducts from the production well are provided on the surface.

In situ combustion has been carried out in the reservoir shown in FIG. 1of the drawing by establishing a combustion front in the vicinity of theinjection well and thereafter injecting air in order to propel the frontthrough the reservoir toward the production well. A number of methodsfor estaolishing such a front are well known, including the injection ofhigh temperature combustion products into the reservoir, the use ofelectrical devices to ignite a mixture of fuel gas and oxygen in theinjection well opposite the producing formation, and the injection ofpyrophoric materials and a stream of combustion-supporting gas into thereservoir through the injection well. As shown in FIG. 1, overburningoccurred as the combustion front thus established progressed through thereservoir. The upper portion of the reservoir was substantially burnedout and thus depleted of oil. The lower section of the reservoir, on theother hand, was relatively unaffected by passage of the combustion frontand hence still contains appreciable quantities of oil. Referencenumeral 15 designates the boundary between the burned and unburnedsections. Breakthrough of the combustion front occurred in theproduction well near the top of the producing zone. As a result of theoverburning and premature breakthrough of the combustion front,continued injection of air or oxygen into the reservoir will result incombustion near or in the production well. A substantial portion of theinjected oxygen will flow through the high permeability burned-out zoneat the top of the reservoir and will not be utilized for the generationof heat within the reservoir. In situ combustion in the section of thereservoir below the burned-out zone will depend largely upon thediffusion of oxygen downwardly into the cooler region at the bottom ofthe reservoir. Since this occurs only to a limited extent, utilizationof oxygen and overall efficiency of the process will be poor.

In carrying out the process of the invention, the injection of air oroxygen at injection well 11 is discontinued upon breakthrough of thecombustion front at production well 14. Imminent breakthrough of thecombustion front can often be detected by observing the production welltemperature. A substantial rise in temperature over a relatively shortperiod of time generally indicates that combustion is occurring in thearea immediately surrounding the production well. The appearance ofoxygen in the product gases indicates that breakthrough has occurred.After air injection has been halted following breakthrough of theinitial front, a secondary combustion front is established in the hotreservoir at an intermediate point between the injection and productionwells by introducing a gas stream containing fuel gas, oxygen and one ormore inert gases through injection well it. The gaseous mixture utilizedto establish the secondary front may comprise, for example, about 3.6volume percent methane, 260 volume percent air, and 70.4 volume percentnitrogen. As pointed out previously, a variety of fuel gases anddiluents may be employed.

The gaseous mixture introduced into the reservoir as described abovewill flow toward the production well, absorbing heat from the formation,until it has reached the ignition temperature. As that point, the fuelgas in the mixture will ignite and combustion will occur. The secondarycombustion front which results is indicated by reference numeral 16 inFIG. 1. Under normal operating conditions this secondary front willgenerally occur at a point where the injected gases have traversed aboutone-fifth of the reservoir volume swept by the previous front. The heattransfer characteristics in oil reservoirs during most in situcombustion operations are such that the gases will usually flow throughabout 20 percent of the swept volume before the fuel ignitiontemperature is reached. Other factors also affect the position of thesecondary front, however, and hence its precise location cannotordinarily be predetermined.

The gases employed to establish and maintain the secondary front willgenerally be injected into the reservoir at temperatures between theambient temperature and about 400 F. The temperature should obviously besomewhat below the ignition temperature of the fuel gas in the mixture.Heat economies can usually be effected by recycling a portion of thecombustion products recovered from the production well to the injectionwell as diluent gases. The pressures at which the gases are introducedmay range from values slightly in excess of the formation pressures upto values approaching pressures at which fracturing of the reservoiroccurs. Pressures between about 100 p.s.i.g. and about 1,000 p.s.i.g.are generally preferred.

Following establishment of the secondary combustion front at anintermediate point within the reservoir as shown in FIG. 1 of thedrawing, the composition of the input gas stream is adjusted until ahigh oil recovery rate is attained at the production well. Heatliberated due to combustion of the fuel gas is carried toward theproduction well be conduction of the hot gases. The temperature of theportion of the reservoir down stream from the combustion front throughwhich the gases flow rises. Overlying and underlying portions of thereservoir are heated by conduction. As heat diffuses into the oil-sat- 5urated rock below the burned zone, the viscosity of the oil is reduced.The oil moves toward the producing Well by the imposed pressure gradientbetween the wells and by the hydrostatic pressure of the oil.

Maximum reservoir temperatures in an in situ combustion process carriedout in accordance with the invention occur within a relatively narrowzone at the leading edge of the secondary combustion front. Thetemperature beyond this zone declines to a relatively low value at theproduction well. The temperature at the production well can readily becontrolled by regulating the volume of gases injected into thereservoir. An increase in the injection rate results in an increase intem perature at the production well; while the temperature declines witha decrease in the injection rate. A change in the injection rate willfrequently result in some move ment of the combustion front even thoughthe composition of the input gas stream remains unchanged. Adjustment ofthe composition to maintain a stationary combustion front may thereforebecome necessary.

A decline in the rate at which oil is recovered from production well 14after the secondary combustion front has been held stationary for sometime indicates that the front should be moved forward in order toincrease the amount of heat transferred to sections of th ereservoirlocated some distance from the injection well. This can be accomplishedby reducing the oxygen content of the gas stream introduced into thereservoir through injection well 11. Since it is generally preferred tomaintain the input gas volume constant while changing the oxygencontent, a portion of the oxygen in the input stream will normally bereplaced by inert gas. Using a mixture of methane, air and nitrogen forexample, the air content might be reduced from about 26.0 volume percentto about 23.0 volume percent while increasing the inert gas content fromabout 70.4 volume percent to about 73.4 volume percent. Such a changecauses the combustion front to move in the direction of production well14. After an increase in the oil recovery rate is noted at well 14, thecomposition is again held constant until a change in recovery rate isobserved. This incremental movement of the secondary front through thereservoir is continued until the reservoir has been depleted of oil.

The system depicted in FIG. 2 of the drawing is similar to that shown inFIG. 1 except that in FIG. 2 the reservoir is a stratified one.Injection well 20 and production well 21 extend downwardly throughoverburden 22 and oil-bearing zones 23, 24 and 25. Zone 24 has greaterpermeability than adjacent zones 23 and 25 and hence the initialcombustion front employed in the reservoir rapidly moved through zone 24and broke through at production well 21 before recovery in the adjacentzone had been completed. Establishment of a secondary combustion frontin the high permeability zone at a point between the injection andproduction wells as indicated in FIG. 2 by reference numeral 26 resultsin the complete utilization of oxygen injected into the reservoir andimproves heat transfer to oil-bearing zones 23 and 25. The secondaryfront may be moved forward in the reservoir at intervals in order toobtain maximum oil recovery rates in the manner described in conjunctionwith FIG. 1 above.

It will be recognized that the method of the invention is applicable toa variety of in situ combustion operations and is not limited to thespecific systems described above. It may, for example, be utilizedfollowing break through of the initial combustion front in a reverseburning operation wherein air is injected in one well and flows throughthe reservoir to a second well at which combustion is initiated. Themethod may be utilized in conjunction with operations for the recoveryof oil from shale deposits wherein the deposit is first fractured and insitu combustion is thereafter utilized to recover oil. Otherapplications will be apparent to those skilled in the art.

What is claimed is:

1. An improved oil recovery process which comprises establishing aninitial combustion front in a subsurface oil-bearing reservoir in thevicinity of a first well penetrating said reservoir; injecting acombustion-supporting gas into said reservoir to advance said initialcombustion front toward a second well penetrating said reservoir;recovering oil and gases from said reservoir; discontinuing theinjection of said combustion-supporting gas upon breakthrough of saidinitial combustion front at said second well; establishing a secondarycombustion front in the burned-out section of said reservoirintermediate said first and second wells by injecting a combustiblemixture of a fuel gas, oxygen and an inert gas into the portion of saidreservoir traversed by said initial combustion front while thetemperature of at least part of said burned-out section remains abovethe ignition temperature of said combustible mixture; adjusting thecomposition of said combustible mixture to maintain said secondarycombustion front essentially stationary within said burned-out sectionand recovering additional oil and gases from said reservoir.

2. A method for improving oxygen utilization during an in situcombustion oil recovery operation which comprises establishing aninitial combustion front in a subsurface oil-bearing reservoir adjacentan injection well penetrating said reservoir; injecting acombustion-supporting gas through said injection well to advance saidinitial combustion front toward a production well penetrating saidreservoir; producing oil and gases from said production well; haltingthe injection of said combustionsupporting gas at said injection wellupon breakthrough of said initial combustion front at said productionwell; injecting a combustible mixture of a fuel gas, oxygen and an inertgas through said injection well while the temperature of at least partof the burned-out zone in said reservoir exceeds the ignitiontemperature of said combustible mixture to establish a secondarycombustion front in said reservoir intermediate said injection andproduction well; adjusting the composition of said combustible mixtureto maintain said secondary combustion front at an essentially stationaryposition within the burned-out zone in said reservoir and producingadditional oil and gases from said production well.

3. A method for reducing the influx of free oxygen into the productionwell during an in situ combustion oil recovery operation which comprisesheating a subsurface oil-bearing reservoir adjacent an injection well toa temperature sufficient to establish an initial combustion front insaid reservoir; injecting a combustion-supporting gas into saidreservoir through said injection well to move said initial combustionfront toward a production well; producing oil and gases from saidproduction well; discontinuing the injection of saidcombustion-supporting gas at said injection well upon breakthrough ofsaid initial combustion front at said production Well; injecting acombustible mixture of a fuel gas, oxygen and an inert gas through saidinjection well while the temperature of at least part of said reservoirremains above the ignition temperature of said combustible mixture toestablish a secondary combustion front in said reservoir intermediatesaid injection and production wells; adjusting the fuel-oxygen ratio insaid injection mixture to maintain said secondary combustion front at anessentially stationary position within the burned-out section of saidreservoir; and producing additional oil and gases from said reservoirthrough said production well.

4. A process as defined by claim 3 wherein said mixture injected intosaid reservoir to establish said secondary combustion front containsfrom about 1 to about 10 volume percent of fuel gas, from about 3 toabout 25 volume percent of oxygen, and from about to about 96 volumepercent of inert gas.

5. A process as defined by claim 3 wherein the oxygenfuel ratio in saidinjection mixture is adjusted at intervals following establishment ofsaid secondary combustion front.

6 A process as defined by claim 3 wherein the gas injection rate isvaried to control the production well temperature followingestablishment of said secondary combustion front.

7. In an oil field secondary recovery operation wherein a combustionfront is established in a subterranean oilbearing reservoir in thevicinity of a first well and propagated through said reservoir toward asecond well and wherein overburning and the formation of a burned-outzone near the upper boundary of said reservoir occur, the improvementwhich comprises injecting a combustible mixture of oxygen, a fuel gasand an inert gas into said reservoir after said combustion front reachessaid second Well and while the temperature of at least part of saidburned-out zone is still above the ignition temperature of saidcombustible mixture to establish a secondary combustion front Withinsaid burned-out zone, adjusting the composition of said combustiblemixturev to maintain said secondary combustion front essentiallystationary within said burned-out zone, and recovering oil from saidreservoir as a result of the generation of heat at said secondarycombustion front.

8. In an oil field secondary recovery operation wherein a combustionfront is established in a subterranean oilbearing reservoir in thevicinity of a first well and propagated through said reservoir toward asecond Well and wherein said combustion front advances through a highlypermeable zone of said reservoir, producing a burned-out zone bounded bya low permeability zone containing substantial quantities of oil, theimprovement which comprises injecting a combustible mixture of oxygen, afuel gas and an inert gas into said reservoir after said combustionfront reaches said second well and while the temperature of at leastpart of said burned-out zone is still above the ignition temperature ofsaid combustible mixture to establish a secondary combustion frontwithin said burned-out zone, adjusting the composition of saidcombustible mixture to maintain said secondary combustion frontessentially stationary within said burned-out zone adjacent said lowpermeability zone, and recovering oil from said low permeability zone asa result of the generation of heat at said secondary combustion front.

References Cited in the file of this patent UNITED STATES PATENTS2,793,697

7. IN AN OIL FIELD SECONDARY RECOVERY OPERATIONS WHEREIN A COMBUSITONFRONT IS ESTABLISHED IN A SUBTERRANEAN OILBEARING RESERVOIR IN THEVICINITY OF A FIRST WELL AND PROPAGATED THROUGH SAID RESERVOIR TOWARD ASECOND WELL AND WHEREIN OVERBURNING AND THE FORMATION OF A BURNED-OUTZONE NEAR THE UPPER BOUNDARY OF SAID RESERVOIR OCCUR, THE IMPROVEMENTWHICH COMPRISES INJECTING A COMBUSTIBLE MIXTURE OF OXYGEN, A FUEL GASAND AN INERT GAS INTO SAID RESERVOIR AFTER SAID COMBUSTION FRONT REACHESSAID SECOND WELL AND WHILE THE TEMPERATURE OF AT LEAST PART OF SAIDBURNED-OUT ZONE IS STILL ABOVE THE IGNITION TEMPERATURE OF